The invention relates to a steel alloy for a low-alloy steel for producing high-strength seamless steel.
In particular, the invention relates to tubing which can also have cross-sections other than circular and which can be used as construction tubing for particularly highly stressed welded steel structures, for example, in the construction of cranes, bridges, ships, hoists and trucks.
Such tubing can, in addition to circular cross sections, also have square, rectangular or polygonal cross sections depending on the requirements and application.
Steel alloys for this type of steel tubing are known, for example, from DE 199 42 641 A1. This conventional steel alloy has, in addition to small added amounts of chromium, molybdenum and vanadium and the absence of nickel, an additional amount of tungsten in a range of 0.30-1.00%, which is particular for a low-alloy steel.
Eliminating the otherwise absolutely necessary nickel and/or at least limiting the nickel content to low concentrations is intended to prevent tacky scale and to improve the surface quality, in particular during warm-pilgering of tubing made from these types of steels, and to avoid the otherwise required expensive finish processing of the surface by cutting.
Construction tubing for the aforementioned applications is subject to very stringent requirements with respect to strength and ductility at low temperatures down to −40° C.
To attain the required properties, the tubes must be hardened and tempered after hot-rolling.
The steel known from DE 199 42 641 A1 as FGS 70 reliably attains all minimum values required for elasticity limit, tensile strength, elongation at rupture and notched bar impact work.
However, the requirements for construction tubing for the aforementioned applications have steadily increased over the past years, so that presently construction tubing meeting the following requirements is increasingly demanded:                elasticity limit 960 MPa,        tensile strength 980-1150 MPa,        notched bar impact work 27 J at −40° C.,        assured general weldability,        low or limited Ni-content.        
The required increase in the strength with a sufficient ductility of the hot-processed seamless tubing for the aforedescribed applications requires the development of new alloying concepts. In particular, conventional alloying concepts do not attain sufficient ductility at low temperatures in the elasticity limit region around 1000 MPa.
The mechanism responsible for increasing the strength, which at the same time also leads to an increase in the ductility, is known to be a decrease in the grain size. The grain size can be reduced, for example, by additionally alloying nickel or molybdenum and the associated reduction of the transformation temperature.
These alloying concepts, however, cause the carbon equivalent to increase and therefore result in poorer weldability. Nickel and molybdenum also significantly increase the alloying costs, while nickel additionally degrades the surface quality of the hot-rolled tubing.
However, raising the carbon content as an obvious possibility for increasing the strength would lead to a deterioration of the ductility and a significant increase of the carbon equivalent.
Vanadium is also used for increasing the strength. This concept is based on the mixed-crystal-hardening of the vanadium and the precipitation of very fine vanadium-carbides during the tempering treatment.
However, the aforementioned alloying concepts were unable to attain the required properties.
A reduction of the grain size for improving the mechanical properties can basically also be achieved by thermo-mechanical treatment.
The specific temperature profile during hot-finishing of seamless tubing, however, does not permit the required reduction in the transformation temperature so that conventional concepts for thermo-mechanical treatment can be applied.
Until now, the required stringent requirements can only be attained with high-alloy steels, which has found no or only limited acceptance in the market due to their high costs.
It is an object of the invention to provide a low-cost steel alloy for a low-alloy steel for producing high-strength weldable seamless steel piping, in particular construction piping, which reliably satisfies the aforementioned minimum requirements with respect to elasticity limit, tensile strength, and notched bar impact work and which in addition ensures good general weldability and which produces optically flawless surfaces during hot-rolling.